1. Field of the Invention
This invention pertains in general to a semiconductor manufacturing process and, more particularly, to a method for obtaining uniform photoresist coatings.
2. Description of the Related Art
Photoresist is a light-sensitive coating used during semiconductor device manufacturing processes, and is applied in liquid form. Typically, a small quantity of photoresist is applied to a wafer surface spun at high speed to spread the photoresist over the wafer surface. A pattern of light is then projected onto the photoresist. For a positive-type photoresist, regions exposed to the projected light may be removed by etchants, whereas regions not exposed to light are hardened and become resistant to etchants. In contrast, regions of a negative-type photoresist exposed to light become resistant to etchants. The steps of applying, exposing, and hardening of photoresist are essential in delineating patterns on the surface of a wafer in a semiconductor device manufacturing process.
During the manufacturing process, the wafer surface may acquire a complicated topography that include ridges and deep trenches. Trenches are often formed to isolate adjacent devices, prevent latch-ups in CMOS circuits, and provide capacitor components in high-density dynamic random-access memory cells ("DRAMs"). Because photoresist fills the trenches when it is applied over the wafer surface, the planarity of the coating referred to herein as uniformity, is difficult to obtain and, in some cases, leads to a dip in the photoresist layer above the trenches. For a DRAM manufacturing process, in particular, such a dip may lead to unacceptable variations in the formation of the device.
One method to minimize photoresist coating nonuniformity adds a second coating of photoresist over the first layer. Although the addition of a second layer may improve the uniformity of the coating surface, such an additional coating is insufficient when device size continues to shrink and the vertical features such as the trenches become comparable in size to horizontal features such as wiring.